The purpose of this invention is to realize a means by which one can remotely detect the vibrations of an object using non-contacting techniques involving a laser beam and an optical detection apparatus. One advantage of this invention is that one can measure very small vibrations, on the order of fractions of an optical wavelength in amplitude, over a frequency range on the order of Hz to MHz or more. Another advantage is that this system enables remote sensing to be realized using eye-safe wavelength beams (1.5 μm). Yet another advantage is that this invention enables one to detect vibrations from non-specular objects (e.g., surfaces with roughness features on the order of optical wavelengths) that reflect an incident optical beam into a diffuse set of angles, resulting in a highly speckled pattern of small spots. Still yet another advantage of this invention is that the receiver possesses a set of high-gain, low-noise optical amplifiers as a front-end means to amplify the diffusely scattered beam that enters the receiver, thereby improving the shot-noise-limited sensitivity by two orders of magnitude relative to a conventional multi-speckle compensated vibrometer.
This invention has potential use for remote sensing of vibrating objects without the need for physically contacting the surface under test. This vibrometer can be used for remote sensing of threats (e.g., a UAV can interrogate objects down on the ground such as tanks, minefields, etc.; or, a ground-based platform, say, with a DEW, can interrogate incoming threats to assess its functionality and operational state). In addition, this vibrometer can be used to interrogate objects on the ground (from an airborne location) to ascertain the nature of the materials that comprise the object in question, and, hence classify the object in terms of its functionality (in this case, a second laser, in conjunction with the device disclosed herein, can be employed to excite photo-acoustic modes in the material under interrogation). In addition, one use such an invention for real-time, manufacturing in situ process control, as well as in-service inspection of materials (structures, welds, bonds, etc.) and life-cycle evaluation of smart materials, the latter case referred to as health monitoring of infrastructures via “inspection on demand”.
The prior art includes single-speckle vibrometers as well as multi-speckle, compensated vibrometers. In the former case, a laser beam impinges on the object under test, and the receiver is designed to receive a single speckle, which is then directed into a coherent detector (either a homodyne or heterodyne system). This system can be shot-noise limited in sensitivity, but, its overall performance is reduced by ≈30 dB over multi-speckle receivers, since it is only capable of processing a single speckle. The present invention enables one to process many speckles, thereby enhancing the performance of the system. In the latter case, there exist a variety of multi-speckle vibrometer devices, such as self-referencing receivers (using a Fabry-Perot resonator as a multi-speckle FM discriminator), as well as single-speckle vibrometers with adaptive optical front-end devices (such as 2-wave mixers, SLMs, photo-emf sensors, etc.). In these cases, multiple speckles can be processed, but, at the expense in terms of sensitivity (the Fabry-Perot), noise and throughput (in the case of 2-wave mixers), and sensitivity in excess of the shot-noise limit (the photo-emf devices). The present invention enables one to realize shot-noise limited sensitivity, with improved performance beyond existing systems (owing to the front-end low-noise amplifiers), with wavefront-compensation capability. The enhancements derive from the fact that the low-noise front-end fiber amplifiers also provide for adaptive optical compensation of wavefront distortions.
One aspect of the prior art involves single-speckle vibrometers. As such, the extension from a single-speckle receiver to one with multi-speckle processing is not an obvious extrapolation. Therefore, it is our contention that this invention is indeed, novel, and, therefore, is not obvious to those skilled in the art. In terms of the prior art in multi-speckle vibrometers, the front-end devices are either totally passive in nature (such as the self-referencing Fabry-Perot resonators) or, at best, active in the sense that a collection of multiple speckles are processed via beam cleanup or real-time holography. In none of these multispeckle-based vibrometers is the notion of front-end amplification discussed or implied, rather the prior art here focuses on methods to deal with the highly diffuse and speckled incident beam, without further active processing. The present invention goes beyond the prior art, in that it not only provides a way of processing a highly speckled incident beam, but, also, at the same time (and, also, inherent in the same process as a matter of fact), this invention adds low-noise gain for enhanced shot-noise limited sensitivity by approximately 20 dB (fiber amplifiers can provide small signal gains at ≈40 dB without parasitics).